1. Field of the Invention
The present invention relates to an iron species preform for forming a metal matrix composite excellent in cast-in performance by an aluminum species alloy used for forming a Metal Matrix Composite (MMC).
2. Related Art
In a background art, for example, in a vehicular engine, there is widely used a cylinder block constituted by casting an aluminum species alloy for achieving a reduction in weight. In such an engine, when a journal portion is formed at the cylinder block made of the aluminum species alloy (thermal expansion coefficient: about 21.0×10−6/° C.) and a crankshaft made of iron species material (thermal expansion coefficient: about 9×10−6 through 12×10−6/° C.) is axially supported by the journal portion by interposing a bearing metal, heat generated by combustion of a mixture gas at inside of a cylinder in operating the engine is transmitted to the journal portion. When a temperature of the journal portion rises, by a difference between the thermal expansion coefficients of the iron species material and a base metal made of the aluminum species alloy, a clearance between a bearing face of the journal portion and the crankshaft interposing a bearing metal becomes excessively large to bring about vibration or noise in running a vehicle.
Hence, for example, in a journal portion for axially supporting a crankshaft of a horizontal opposed 4 cylinder engine, at the journal portion having a bearing face constituted by recessing respective center portions of left and right cylinder blocks made of an aluminum species alloy in a shape of a semicircular arc, by constituting MMC by an iron species preform by casting-in the iron species preform made of an iron species powder sintered member in the cylinder block, a thermal expansion coefficient necessary for the journal portion is achieved without changing the aluminum species alloy of the cylinder block constituting the base material.
However, when a portion or a total of a cast product of an aluminum species alloy is constituted by MMC, it is extremely difficult to ensure a bonding strength of an interface by using generally a casting method, particularly, a High Pressure Die Casting (HPDC) and stably ensure adherence by easily melting the aluminum species alloy to invade a preform comprising an iron species powder sintered member. Further, it is known that when the preform structured by the iron species powder sintered member is inserted in an aluminum species alloy so as to be cast-in, a state of the melted aluminum species alloy invading the preform after cast-in step effects a significant influence on a mechanical property or a physical property, and a casting condition is frequently restricted in order to reduce such an influence.
Further, JP-A-2004-204298 discloses that wettability of the preform made of the iron species powder sintered member and the melted aluminum species alloy and the cast-in performance of the aluminum species alloy are improved, by constituting a structure of the preform made of an iron species powder sintered member to a structure in which a free Cu phase is dispersed in a matrix and by subjecting a shot blasting processing or a steaming processing to constitute a surface roughness of the preform by a specific roughness range, so that a bonding strength between the cylinder block main body made of the aluminum species alloy and the iron species preform is improved.
According to JP-A-2004-204298, the strength of the preform structured by the iron species powder sintered member can be increased by melting Cu to constitute a solid solution and the bonding strength of the interface can be increased by reacting with the aluminum species alloy when the preform is cast-in with the aluminum species alloy by being precipitated into the matrix as the free Cu phase.
However, when the preform is cast-in with the aluminum species alloy, there is a concern that before the interface between the preform and the base material reaches a constant bonding strength when a stress generated in solidifying and shrinking the melted aluminum species alloy to which the preform is inserted, the adherence of the interface becomes unstable and a clearance is generated at the interface to make the bonding strength unstable, owing to a shape, a specification or the like of the preform. A phenomenon of instability of the bonding strength at the interface and production of the clearance or the like is significant when accompanied by rapid cooling and solidification as in HPDC.
When there is present a clearance at the interface of the base material comprising the aluminum species alloy and the preform in the journal portion, a heat conduction efficiency between the base material and the preform is reduced to bring about a dispersion in the thermal conductivity in a peripheral direction of the journal portion. The journal portion is not uniformly expanded by the dispersion, support of the bearing metal by the bearing face of the journal portion becomes unstable and a friction coefficient between the crankshaft and the bearing metal is increased. Owing to the increase in the friction coefficient, that is, an increase in a friction resistance, wear of the bearing metal is significantly increased to constitute a factor of deteriorating fuel cost, performance, durability or the like of the engine.
Further, when there is the clearance at the interface between preform and the base material at the journal portion, in machining the bearing face of the journal portion, the portion formed by the thin wall is elastically deformed by a load in machining to deteriorate machining accuracy of the journal portion.
Further, when the clearance is present at the interface, stress concentration by high load is brought about by a residual stress or a difference of thermal expansion generated in solidifying and shrinking the melted aluminum species alloy into which the preform structured by the iron species powder sintered member is cast-in. In addition, there is a case in which the aluminum species alloy portion, that is, the base material into which the preform is cast-in is destructed.